Nozzles formed in monocrystalline silicon

ABSTRACT

Method for producing a predetermined pattern of small size fluid nozzles of identical or different geometries in crystallographically oriented monocrystalline silicon or similar material utilizing anisotropic etching through the silicon to an integral etch resistant barrier layer heavily doped with P type impurities.

United States Patent [191 Bassous l l NOZZLES FORMED IN MONOCRYSTALLINE SILICON [75] Inventor: Ernest Bassous, New York City,

[73] Assignee: International Business Machines Corporation, Armonk, N.Y.

[22] Filed: Dec. 31, 1974 [21] Applv No.: 537,799

[52] US. Cl. 239/601; 239/102; 239/602; 239/DlG. 19; 346/75 [51] Int. Cl. ..B05B1/08;GO1D 15/18 [58] Field of Search 239/4, 102,602, 601. 239/DIG. l9;346/1, 75, 140

[56] References Cited UNITED STATES PATENTS 2,665,946 l/l954 Broughton 239/601 X 2,834,635 5/l958 Miller 239/602 X 1 Nov. 25, 1975 2,987,262 6/1961 Goyette et al. 239/596 X 3,125,295 3/l964 MOSS et ul 239/4 X 3,211,088 10/1965 Naiman..... 239/]02 X 3,655,530 4/1972 Tayl0r.... 346/75 X 3,657,599 4/l972 Kashio 346/75 X OTHER PUBLICATIONS IBM Technical Disclosure Bulletin, Vol. 17, No. 3, Aug. 1974, High-Tolerance Control...Nozzles, L. Kuhn et al., 2 pages (pp. 928 and 929).

Primary E.raminerRobert S. Ward, Jr. Attorney, Agent, or FirmJack M. Arnold [57] ABSTRACT Method for producing a predetermined pattern of small size fluid nozzles of identical or different geometries in crystallographically oriented monocrystalline silicon or similar material utilizing anisotropic etching through the silicon to an integral etch resistant barrier layer heavily doped with P type impurities.

4 Claims, 9 Drawing Figures NOZZLES FORMED IN MONOCRYSTALLINE SILICON BACKGROUND OF THE INVENTION The following literature references and patents disclose the employment of anisotropic or preferential etchants with semiconductor materials:

U.S. Pat. No. 3,725,160 to Bean et al.; U.S. Pat. No. 2,770,533 to W. K. Zwicker; U.S. Pat. No. 3,746,587 to Rosvold; U.S. Pat. No. 3,742,317 to Shao; Great Britain Pat. No. 869,669; J. Electrochemical Society, 114, 965 (1967) by Finne and Kline; .I. Electrochemical Society, 118, 401 (1971) by Bohg; J. Electrochemical Society, 111, Abstract 89, 202 (1962) by Crishal and Harrington; J. Electrochemical Society, 1 16, 1325 (1969) by Greenwood; J. Applied Physics, 40, 4569 (1969) by Lee; RCA Review, p. 271 (June 1970) by Stoller; .l. Electrochemical Society, 119, 1769 (1972) by Sedgwick, Broers and Agule, as well as others.

In the prior art, single fluid nozzles or arrays of fluid nozzles, which for example may be used in ink jet printers, were generally made of tubes which are single structures. These nozzles were formed by drilling holes in plates by mechanical or electromechanical means, by the use of an electron beam or a laser or the like. These plates are formed for example of stainless steel, glass or quartz, vitreous carbon, jewels such as sapphire and the like. The techniques set forth above suffer from at least some of the following disadvantages, namely (1) generally a single nozzle is formed, (2) the control of the individual size of nozzles is relatively poor, (3) fabrication of arrays of such nozzles is even more difficult, with attendant nonuniformity of size of holes and spatial distribution of the array. In ink jet printing applications, a jet of ink is forced through a vibrating nozzle causing the jet of ink to break up into droplets of equal size. Printing is effected by controlling the flight of the droplets to a target such as paper. Important characteristics for ink jet printing applications are, the

size of respective nozzles, spatial distribution of the nozzles in an array, and the means for vibrating the respective nozzles. Such factors affect velocity uniformity of fluid emitted from the respective nozzles, directionality of the respective droplets, and break off distance of the individual droplets, that is the distance between the exit of the nozzle and the position of the first droplet.

According to the present invention individual nozzles or an array of such nozzles may be batch fabricated easily due to the crystallographic perfection of the starting material, namely the semiconductor used, which for example may be silicon and the selectivity of the etchant. There is a high degree of control of nozzle size resulting from precise control of processes used in fabrication, namely the formation of diffused layers with required dopant concentrations; control of etch rates of semiconductor material as a function of its crystallographic orientation and of its conductivity type and dopant concentration. In addition, the etch rate of anisotropic etching solutions is controlled as a function of their composition and temperature and the process environmental characteristics. In the fabrication of arrays, the same degree of control is obtainable as for a single nozzle, as is the control of spatial distribution and uniformity of hole size due to high control of the photolithographic process.

The orifice like properties of the instant nozzles are better than those of pipes because wall effects are minimized. Since the nozzle, according to the present invention, is tapered from the entrance orifice to the exit orifice, the wall effects are substantially reduced.

Another advantage of the nozzle of the present invention is that inspection of a given nozzle may be done visually and such inspection is sufficient to anticipate the performance of the individual nozzle. That is, the nozzle is inspected for orifice size and integrity of the structure, without having to actually check the performance of the nozzle in an ink jet printer.

The nozzle of the present invention may pass fluid in either direction, but in the preferred mode of operation fluid flow is in the direction of the larger opening to the smaller opening of the nozzle since there is less pressure drop.

The present invention employs anisotropic etching of monocrystalline silicon, crystallographically oriented, in order to produce one or a plurality, i.e., an array, of identically or unidentically shaped nozzles or holes in a thin monocrystalline semiconductor wafer. The holes are generally of 25 micrometers or less in diameter at the surface of the wafer and are positioned in a background window or membrane due to the anisotropic etching effect on the underlying crystallographically oriented monocrystalline silicon. Depending upon masking technique and selection of plane of the crystallographically oriented material, geometrical design of the hole may be varied in a predetermined manner, i.e., triangles, rectangles or squares may be produced in place of circular holes.

By practice of this invention one is able to produce a pattern of identical holes of small size in a semiconductor wafer. The wafer may then be employed as a nozzle in process and/or apparatus designs requiring the feeding of fluid through holes of 25 micrometers or less in diameter, such as in magnetic and electrostatic ink jet processes, and other gas or liquid metering and filtering systems requiring calibrated single or multiple orifices. Further, the wafer produced by the process of this invention is characterized by a pattern of a plurality of orifices which may also be employed as a substrate for wiring and packaging integrated circuits and other solid state components, or a filter or guide for electromagnetic radiation.

SUMMARY OF THE INVENTION It is an object of this invention to provide a process for producing a single nozzle or an array of fluid nozzles in a semiconductor wafer.

Another object of this invention is to provide a process for producing a single hole or a plurality of holes of predetermined shape and size in a semiconductor wafer of crystallographically oriented monocrystalline silicon for forming nozzles.

A further object of this invention is to provide a process for producing holes of diminishing cross-section, from one side to the other, through a thin semiconductor wafer for forming nozzles.

The above, as well as other objects which will be apparent to the skilled artisan are provided by a process for producing nozzle(s) comprising one of a plurality of apertures in a thin crystallographically oriented non-p monocrystalline silicon material comprising forming an aperture mask on one surface of the silicon material, forming a p surface layer on the non-masked areas of said one surface, anisotropically etching a tunnel from 3 the opposite surface of said silicon material through to the masked area of said one surface and removing the mask. Alternatively, a p layer is formed on a surface, a tunnel is anisotropically etched from another surface to said p layer and the area of the p layer over said tunnel corresponding to the aperture is removed.

In preferred embodiments of this invention the p layer is formed by diffusion of ion implantation into or epitaxial growth on the surface of the monocrystalline silicon body. Preferred plane orientations for the monocrystalline silicon are to provide preferential etching along the (100), (1 l) and (l 1 l) oriented silicon planes.

In order to ensure the effective termination of etching at the p layer, the p barrier layer is defined as containing a p-type dopant atom concentration l0 cm preferably a concentration 3 7 X lO cm DESCRIPTION OF THE DRAWING FIGS. 1-4 represent sequential cross-sectional views of a silicon wafer processed in accordance with this invention;

FIGS. 5 and 6 illustrate front and cross-sectional views of a nozzle produced in accordance with the sequence illustrated by FIGS. 1-4.

FIGS. 79 represent sequential cross-sectional views of a silicon wafer processed by another example of the process of this invention.

DETAILED DESCRIPTION OF THE INVENTION For a variety of uses it is desirable to provide one or more precise, reproducible apertures in or through a monocrystalline silicon wafer. The present invention is a process for chemically drilling a hole through monocrystalline silicon using an anisotropic etchant for forming a fluid nozzle or an array of such fluid nozzles.

Anistropic or preferential etchants attack solid materials in different directions at different rates. Numerous anisotropic etchants are known for monocrystalline silicon which include alkaline liquids or mixtures thereof. As common single crystal silicon anisotropic etchants there may be mentioned aqueous sodium hydroxide, aqueous potassium hydroxide, aqueous hydrazine, tetramethyl ammonium hydroxide, mixtures of phenols and amines such as a mixture of pyrocatechol and ethylene diamine with water, and a mixture of potassium hydroxide, n-propanol and water. These and other preferential etchants for monocrystalline silicon are useable in the process of the present invention for forming fluid nozzles.

Although it is known that the rate of preferential etching varies with respect to chemical constituents of the etchant, types of silicon and specific impurities therein, temperature and concentration of etchant, particular crystallographic orientation of the single crystal silicon and other factors, it is known that etching virtually ceases at a p barrier layer. Thus by forming a p yp -P yp P" yp f r p n yr -1 yp type-p type, or n type-p type junction in monocrystalline silicon, etching action of the anisotropic reagent is effectively retarded or completely stopped at the junction.

With respect to the three most common low index crystal planes in single crystal silicon, anisotropic etch rate is greatest for (100) oriented silicon, somewhat less for (110) and is least for (111) oriented silicon. Further with n-type silicon or p-type silicon with an impurity dopant concentration of less than about l0cm etch rate does not vary significantly, other factors remaining constant. However, with p-type dopant concentration l0 cm the p-type silicon would be known as p type silicon to the skilled artisan. From a practical standpoint, in order to ensure an adequate p layer to regulate etch rate, p-type impurity is applied to the saturation point of the surface area of the silicon body.

FIGS. 1, 2, 3 and 4 illustrate one exemplary sequence of process steps to produce an aperture or hole in a single crystal silicon wafer for forming a fluid nozzle. It is to be appreciated that the following process steps may be used in a different sequence. Also, other film materials for performing the same function below may also be used. Further, film formation, size, thickness and the like may be varied. The wafer of single crystal silicon 1 is of oriented silicon, p-type, about 7.5 8.5 mils thick. Front and back surfaces 3 and 5, respectively, are mechanically chemically polished using known techniques. On the front side a first silicon nitride film 7, 500 A. thick is deposited followed by a second layer 9 of silicon dioxide, 4000 A. thick, on top of the silicon nitride. The silicon nitride may be omitted if the silicon dioxide is made thicker to act as a mask for acceptor type impurities. Thereafter, the wafer is thermally oxidized, for example, in steam at 1000C., to grow a silicon dioxide film 11, 5000 A. thick, on the back of the wafer. The wafer at this stage is shown by FIG. 1.

Next, by a photolithographic process, a pattern is defined on the surface of layer 9 to allow removal of silicon dioxide and silicon nitride from surface 3, except in the areas where apertures or holes are required in the final nozzle structure. In accordance with the pattern, the area of silicon dioxide-silicon nitride layer on the first surface 3 is removed by standard etching techniques except at those areas such as 13 which are masked in accordance with the pattern. The wafer at this point in the process is shown by FIG. 2.

Thereafter, acceptor-type impurities such as boron, gallium, aluminum or the like are introduced into front surface 3 to produce a p* layer. This layer is as close to saturation as possible. One convenient way to achieve a p* layer is to use a boron tribromide source. Drive-in of the dopant impurity is accomplished in known manner by heating in a nitrogen atmosphere at temperatures in excess of 1000C. Silicon dioxide is then grown or deposited over the wafer surface. In FIG. 3, it is seen that a p layer 15 is formed at the surface of the initial silicon wafer 1, and a layer of silicon dioxide 17 is deposited over the unmasked portion of front surface 3, as well as the exposed regions of masking layers 7 and 9.

A second photolithographic step is now performed on the back or opposite side of the wafer aimed at exposing the silicon surface in areas opposite to and in alignment with area 13. An alignment tool capable of front to back alignment is required for this step. Then a three dimensional tunnel is etched through the silicon, by using one of the previously mentioned anisotropic etchants, from the back surface through to the silicon nitride mask 7 since the mask 7 has prevented the formation of a p area corresponding to area 13 thereunder. The wafer at this point is illustrated by FIG. 4. Thereafter, the remainder of masking layer 7 and associated layer 17 are removed, such as by use of a suitable etchant. Assuming that the alignment pattern was selected to provide a circular silicon nitride mask prior to formation of the p surface layer, the result is a circular hole 20 centrally positioned within a square silicon windowpane 22 within (100) oriented silicon. This result is illustrated in front view by FIG. 5 and in cross-section by FIG. 6. Tunnel 21 is shaped in the form of a regular truncated rectangular or square pyramid.

Typical physical dimensions of a single nozzle fabricated in (100) oriented single crystal silicon wafer 200 micrometers thick are an entrance aperature approximately 325 micrometers on each side of the square tapered to a square membrane 22 approximately 50 micrometers on each side. The'thickness of the membrane is variable in the range of 1-1 0 micrometers. The size of the hole in the membrane is on the order of 25 micrometers diameter.

Typical characteristics of the described nozzle in ink jet printing applications are as follows. At fluid pressures up to 80 pounds per square inch the break-off uniformity of an array of nozzles, for example eight nozzles, is less than one-half a wavelength. Velocity uniformity is better than 21%, and the directionality,

that is the directional alignment of the respective fluid jets, is within :1 milliradian of parallel alignment. The efficiency of this tapered nozzle is superior to tubular nozzles as distinguished by the minimal drop in fluid pressure from the entrance orifice to the exit orifice.

Another embodiment of this invention, illustrating the principles thereof, is as follows. Initially, a silicon dioxide layer 25 is formed on the back surface of( 100) oriented single crystal silicon wafer 27. Thereafter, a p surface layer 29 is formed to saturation, for example, by a p" diffusion, ion implantation or epitaxial deposition on the front surface of the silicon wafer. The wafer at this point of the process is illustrated by FIG. 7.

Subsequent to the formation of the p layer a masking film such as silicon dioxide or silicon nitride is grown or deposited on the p layer. Such a film is shown as 35 on FIG. 8.

Thereafter, openings are etched in the areas of masking film 25 followed by the etching of tunnels 31 with an anisotropic etchant through to the p" junction to form silicon window panes 33. The wafer is now illustrated by FIG. 8. Thereafter, openings are etched in the areas of masking film 35 where apertures are to be formed, such as at areas 37, exposing the p layer at these locations, FIG. 9. Such openings 37 are required to be aligned relative to areas 33 (FIG. 9) using a front to back alignment tool.

To complete the process, the p layer is then etched through to form the hole or aperture 39 in the silicon window by using any known technique, such as an isotropic etchant (for example, a mixture of hydrofluoric, nitric and acetic acids), electrolytic etching, ion etching or sputtering in a partial vacuum, laser or electron beam etching, n diffusion or n ion implantation followed by anisotropic etching and the like procedures. This etching process is performed after protecting the inner surfaces of tunnel 31, including surface 33.

A preferred anisotropic etchant composition useable in the above examples is:

Pyrocatcchol 4 grams Ethylene diaminc 25 ml Water 8 ml at l 18 i 1C.

Although the invention has been illustrated with oriented silicon, single crystal silicon of other orientation may be used, but the three-dimensional geometry of the etched cavity will be different, but equally uniform for a given material. Where desired, the three-dimensional structure may be controlled in numerous ways, such as taper shape, shape of inner window and outer orifices, use of a plurality of tunnels leading to a single orifice, etc. A plurality of apertures may be formed in square or rectangular windows. In the case of silicon, L shaped, U shaped and square frame shaped windows may be used. Further, mesa structures of different heights may be fabricated, each mesa being characterized by its own orifice or array of orifices. Also other semiconductor materials exhibiting the same crystallographic properties and selective etching properties may be used. Such other semiconductor materials include germanium and compound semiconductors such as those from group III and V elements of theperiodic table of elements, for example, gallium arsenide. As noted above, one use of this invention is to produce a nozzle plate of precisely calibrated orifices to control gas and liquid flow, particularly for ink jet printing.

Although the invention has been illustrated in detail with the employment of silicon nitride and silicon dioxide masking layers, other equivalent layers such as aluminum oxide, are applicable. Phosphoric acid is often used to dissolve silicon nitride while dilute and buffered hydrofluoric acid dissolves the above oxide masking layers. Other isotropic etchants for p silicon if the embodiment of FIGS. 7-9 is employed, are mixtures of oxidizing agents such as hydrogen perioxide, nitric acid, or potassium permanganate and hydrofluoric acid.

Variation of the invention will be apparent to the skilled artisan.

What is claimed is:

1. A nozzle comprising:

a nozzle body formed of a semiconductor material having a rectangular entrance aperture of a first cross-sectional area which tapers to a second crosssectional area which is smaller than the cross-sectional area of said entrance aperture; and

a membrane of said semiconductor material formed within said second cross-sectional area and having an exit aperture formed therein having a smaller cross-sectional area than said second cross-sectional area and having a different cross-sectional geometry than said second cross-sectional area.

2. The combination claimed in claim 1 wherein said semiconductor material is monocrystalline silicon.

3. The combination claimed in claim 2 wherein said exit aperture is substantially circular in cross-section.

4. The combination claimed in claim 3 wherein said entrance aperture and said second cross-sectional area are substantially square in cross-section. 

1. A NOZZLE COMPRISING A NOZZLE BODY FORMED OF A SEMICONDUCTOR MATERIAL HAVING A RECTANGULAR ENTRANCE APERTURE OF A FIRST CROSS-SECTIONAL AREA WHICH TAPERS TO A SECOND CROSS-SECTIONAL AREA WHICH IS SMALLER THAN THE CROSS-SECTIONAL AREA OF SAID ENTRANCE APERATURE; AND A MEMBRANE OF SAID SEMICONDUCTOR MATERIAL FORMED WITHIN SAID SECOND CROSS-SECTIONAL AREA AND HAVING AN EXIT APERTURE FORMED THEREIN HAVING A SMALLER CROSS-SECTIONAL AREA THAN SAID SECOND CROSS-SECTIONAL AREA AND HAVING A DIFFERENT CROSS-SECTIONAL GERMETRY THAN SAID SECOND CROSS-SECTIONAL AREA.
 2. The combination claimed in claim 1 wherein said semiconductor material is monocrystalline silicon.
 3. The combination claimed in claim 2 wherein said exit aperture is substantially circular in cross-section.
 4. The combination claimed in claim 3 wherein said entrance aperture and said second cross-sectional area are substantially square in cross-section. 